(1) Field of the Invention
The present invention relates to a continuous molten metal plating apparatus and a die casting machine, and, more particularly, to a roller bearing apparatus for continuous molten metal plating bath, a sleeve and a tip in the plunger portion of a die casting machine and composite member and sliding structure for use in the apparatus, the continuous molten metal plating apparatus exhibiting an excellent characteristic against corrosion occurring due to the molten metal and against abrasion occurring due to the load from the roller shaft.
(2) Description of the Prior Art
Hitherto, stainless steel, high chrome steel and hard metal alloy exhibiting excellent corrosion resistance have been used, in the form of built-up welding or a sleeve, in a roller bearing for continuous molten metal plating bath. However, a problem arises in that any of the above-described materials have encountered abrasion or damage in, for example, a zinc plating bath in about one week. As a result, backlash takes place between the roller shaft and the bearing, causing the plating apparatus to be vibrated, with the result that plating characteristics are critically deteriorated. It has been found that the reason for this is that the corrosion of stainless steel, high chrome steel, hard metal alloy or the like, which though exhibits a relatively excellent corrosion resistance, due to the molten metal cannot be prevented completely. Thus, when the roller bearing slides, there occurs at the same time both friction and corrosion abrasion due to the molten metal, so that the quantity of the abrasion is increased. In particular, it has also been found that if the abrasion progresses to a certain degree, corrosion pits occur in the surface of the roller shaft and is the surface of the roller bearing, further increasing the frictional abrasion.
In order to prevent the abrasion of the roller bearing, a material having an excellent corrosion resistance against the molten metal must therefore be selected. Some types of ceramics exhibit a characteristic capable of substantially resisting the corrosion due to the molten metal. Therefore, it is preferable that the ceramics of the type described above be employed as a material for making the roller bearing for the molten metal plating bath.
An example of a technology, in which ceramics are employed in a material for making the roller bearing for the continuous molten metal plating bath, has been disclosed in Japanese Utility Model Laid-Open No. 63-73349. According to the above-described publication, a metal sleeve having the outer surface to which ceramics are applied via a heat resistant buffer sheet is fitted to the roller shaft. Furthermore, the outer surface of the above-described metal sleeve is held by a bearing metals the inner surface of which is coated with ceramics via another heat resistant buffer sheet.
However, the above-described conventional structure has encountered problems in that seizure occurs due to the sliding of the ceramic members and cracks and fractures of the ceramics occur due to the non-uniform contact of the ceramic members because the types of ceramics and the combination of the types of ceramics have not been taken into consideration in the prior art.
That is, although ceramics have an excellent corrosion resistance against molten metal, its wettability is insufficient. It has been found that no lubrication is performed by the molten metal on the sliding surface, and dry abrasion thereby occurs case where ceramics are employed as the material for making a sliding member of the roller bearing for the molten metal plating bath. As a result of a study of the friction coefficients and the threshold surface pressure occurring between typical ceramics, it has been found that the ceramics are apt to encounter cracks and fractures due to seizure and galling if the surface pressure exceeds the threshold level. The reason for this lies in that the friction coefficient is too high to be used as sliding members, and the threshold surface pressure is too low with respect to a combination of silicon carbide and silicon nitride ceramics which have been known to have excellent corrosion resistance and abrasion resistance. It has been considered that the sliding surfaces being in uniform contact with each other cannot be manufactured even if the accuracy for machining the sliding surfaces is improved as much as possible, in the case of ceramics having a high Young's modulus, with the result that local friction occurs due to a slight undulation or an eccentricity. Therefore, when ceramics are employed to make the roller bearing for the molten metal plating bath, the kind of the ceramics must be selected to have both an excellent corrosion resistance against molten metal and a small friction coefficient as well as corrosion against molten metal. In addition a uniform contact sliding surface must be formed through the initial abrasion by the selection of the ceramics.
Furthermore, in the publication of the conventional technology no method of coating the ceramics is disclosed. In addition, the stress occurring when using the roller bearing and fitting the ceramics has not been satisfactorily removed, causing the ceramics to encounter cracks or the like.
That is, according to the above-described conventional structure, a heat resistant material such as an alumina sheet or carbon fiber is used as a stress buffer at the time of the fitting of the ceramics. However, since the above-described heat resistant material has poor elastic deformation capability and plastic deformation capability and since deformation stress exceeds the bending and the tensile strength of the ceramics, there are obtained no effect desired for the stress buffer. In addition, since the ceramics are integrally formed, the structure thus constituted is apt to be fractured by the load acting on the bearing or by the bending stress due to the thermal deformation in the case where the roller shaft is a large size.
Hitherto, stainless steel, high chromium steel and metal materials subjected to nitriding have been employed as the sleeve and the tip of the plunger portion of a die casting machine since the above-described material has a relatively excellent corrosion resistance. However, in a case of, for example, an Al-die casting machine, these materials encounter a problem in that the tip is worn and damaged in about one month and the sleeve is worn and damaged in about three months. As a result, backlash occurs between the sleeve and the tip, causing such problems as penetration of the molten Al and an inferior run of the molten Al. Furthermore, a large quantity of lubricating oil has been used for the purpose of preventing the abrasion damage and galling, however, there occur mold cavities due to the containment of the lubricating oil.
It has been found that the reason for the above-described abrasion damage lies in that corrosion of metal having a relatively excellent corrosion resistance due to the molten metal cannot be completely prevented. Therefore, at the time of sliding of the sleeve and chips, there occur at the same time both the abrasion due to the corrosion caused by the molten metal enhances the abrasion due to the friction, with the result that the quantity of the abrasion is increased.
Therefore, a material having an excellent corrosion resistance against molten metal and excellent abrasion resistance must be selected in order to reduce the quantity of abrasion at the time of the sliding of the sleeve and the tip. In order to meet the above-described conditions, it is preferable that ceramics be selected.
A structure in which ceramics have been used in the sliding portion of the sleeve and the tip of the plunger portion of a die casting machine has been disclosed in Japanese Utility Model Laid-Open No. 63-106561 and another structure has been disclosed in Japanese Utility Model Laid-Open No. 1-127649. The structures thus disclosed are characterized in that ceramic sleeves are shirinkage-fitted to the inner surfaces of the metal sleeves.
However, another problem arises in that the abrasion due to a lapse of time cannot be satisfactorily minimized by using ceramics to make the sleeve and the tip for the purpose of improving the corrosion resistance against the molten metal and the abrasion resistance.
Furthermore, according to the above-described conventional structure, no means has not been employed so as to remove the stress occurring at the time of fitting the ceramic sleeve onto the metal sleeve. Therefore, there has been fear of problems in terms of cracks and fracture of the ceramic sleeve. That is, according to the above-described conventional structure, the ceramic sleeve is directly shrinkage-fitted to the metal sleeve. Therefore, a local excessive stress is apt to occur due to a slight eccentricity and undulation caused at the time of machining the ceramic sleeve and the metal sleeve. As a result, a problem arises in that the ceramic sleeve is apt to be cracked or fractured. In addition, cracks or fracture is apt to occur by the bending stress caused by the thermal deformation or the like during a heat cycle at the time of the usage.